Bacterial virulence factors directly mediate host-pathogen interactions, thus providing tools to probe virulence mechanisms and to identify key host immune functions. Mycobacterium tuberculosis (Mtb) is able to survive intracellularly through extensive manipulation of the host. Mtb produces two secreted virulence factor phosphatases, PtpA and PtpB. PtpB is essential for the survival of Mtb in the host, but its molecular functions are unknown. We now show that PtpB is not a protein tyrosine phosphatase as previously assumed, but has strong similarity to lipid phosphatases, offering novel experimental routes to define PtpB substrate(s) and function. To understand how PtpB mediates Mtb survival in an infected host, we will identify the host substrates of PtpB by substrate trapping, as well as lipidomics and transcriptomics using genetic and chemical PtpB knockouts. Combining these global approaches, we will broadly capture PtpB's effect on the cell and identify PtpB-dependent changes in host lipids. Together, these studies offer a new route towards the understanding of Mtb host manipulation through the essential virulence factor PtpB. Because PtpB is emerging as a novel therapeutic target, these studies will also provide the framework for advancing drug leads.